Spatial Light Modulators (SLMs) have found numerous applications in the areas of optical information processing, projection displays, video and graphics monitors, televisions, and electrophotographic printing. SLMs are devices that modulate incident light in a spatial pattern to form an image corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. The light modulation may be achieved with a variety of materials exhibiting various reflective, refractive, diffractive, electro-optic or magneto-optic effects, or with materials that modulate light by surface deformation.
An SLM typically includes an area or linear array of addressable picture elements (pixels). Using well-known algorithms, source pixel data (e.g., data representing an image) is formatted by an associated control circuit and loaded into the pixel array using any of a number of well-known addressing schemes, typically addressing all pixels in parallel.
One type of SLM, referred to herein as a micro-mirror array, is a monolithic integrated circuit with an array of movable micro-mirrors fabricated over the requisite address, control and drive circuitry. Micro-mirrors are normally bistable, switching between two stable positions in response to digital control signals. Each mirror in a given array forms one pixel, wherein a source of light directed upon the mirror array will be reflected in one of two directions depending upon the selected one of the two stable mirror positions. In an “on” mirror position, incident light to a given mirror is reflected to a projector lens and focused on a display screen or a photosensitive element of a printer; in an “off” mirror position, light directed on the mirror is deflected to a light absorber outside of the numerical aperture of the projecting lens.
When the micro-mirror array is used in a display, the projector lens magnifies the modulated light from the pixel mirrors onto a display screen. Gray scale of the pixels forming the image is achieved by pulse-width modulation, as described in U.S. Pat. No. 5,278,652, entitled “DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System,” which is incorporated herein by reference.
For more detailed discussions of conventional micro-mirror devices, see the following U.S. patents, each of which is incorporated herein by reference:                1. U.S. Pat. No. 5,535,047 to Hornbeck, entitled “Active Yoke Hidden Hinge Digital Micro-mirror Device;        2. U.S. Pat. No. 5,079,544 to DeMond, et al, entitled “Standard Independent Digitized Video System”; and        3. U.S. Pat. No. 5,105,369 to Nelson, entitled “Printing System Exposure Module Alignment Method and Apparatus of Manufacture.”        
The evolution and variations of the micro-mirror devices can be appreciated through a reading of several issued patents. The “first generation” micro-mirror based spatial light modulators were implemented with analog control of electrostatically driven mirrors using parallel-plate configurations. That is, an electrostatic force was created between the mirror and the underlying address electrode to induce deflection thereof. The deflection of these mirrors can be variable and operate in the analog mode, and may comprise a leaf-spring or cantilevered beam, as disclosed in the following U.S. Patents, each of which is incorporated herein by reference:                1. U.S. Pat. No. 4,662,746 to Hornbeck, entitled “Spatial Light Modulator and Method”;        2. U.S. Pat. No. 4,710,732 to Hornbeck, entitled “Spatial Light Modulator and Method”;        3. U.S. Pat. No. 4,956,619 to Hornbeck, entitled “Spatial Light Modulator”; and        4. U.S. Pat. No. 5,172,262 to Hornbeck, entitled “Spatial Light Modulator and Method.”        
This first generation micro-mirror can also be embodied as a digital or bistable device. The mirror is supported by a torsion hinge and axially rotated one of two directions 10 degrees, until the mirror tip lands upon a mechanical stop, or “landing pad.” Such an embodiment is disclosed in U.S. Pat. No. 5,061,049 to Hornbeck entitled “Spatial Light Modulator and Method,” which is incorporated herein by reference. To limit the static friction (stiction) force between the mirror tips and the landing pads, the landing pads may be passivated by an oriented monolayer formed upon the landing pad. This monolayer decreases the stiction forces and prevents the mirror from sticking to the electrode. This technique is disclosed in U.S. Pat. No. 5,331,454 to Hornbeck, entitled “Low Reset Voltage Process for DMD,” and also incorporated herein by reference.
A “second generation” of micro-mirror device is embodied in U.S. Pat. No. 5,083,857 entitled “Multi-Level Deformable Mirror Device,” and U.S. Pat. No. 5,583,688 entitled “Multi-level Digital Micro-mirror Device,” both of which are incorporated herein by reference. In this second generation device, the mirror is elevated above a “yoke,” this yoke being suspended over the addressing circuitry by a pair of torsion hinges. An electrostatic force is generated between the elevated mirror and an elevated electrode, again with parallel-plate actuator configuration. When rotated, it is the yoke that comes into contact with a landing electrode: the mirror tips never come into contact with any structure. The shorter moment arm of the yoke, being about 50% of the mirror, allows energy to be more efficiently coupled into the mirror by reset pulses due to the fact that the mirror tip is free to move. Applying resonant reset pulses to the mirror to help free the pivoting structure from the landing electrode is disclosed in U.S. Pat. No. 5,096,279, entitled “Spatial Light Modulator and Method,” and U.S. Pat. No. 5,233,456 entitled “Resonant Mirror and Method of Manufacture,” both of which are incorporated herein by reference. However, some of the address torque generated between the mirror and the elevated address electrode is sacrificed compared to the first generation devices because the yoke slightly diminishes the surface area of the address electrode.
Despite the aforementioned advances, parallel-plate electrostatic devices generate very low deflection torque and require very low stiffness suspension hinges. Consequently, conventional micro-mirrors are relatively fragile and difficult to fabricate, and may therefore suffer from low yield and increased manufacturing expense. Also, while various process techniques have been developed to ameliorate the stiction problem, the repeated physical contact between the moveable and fixed surfaces still reduces device reliability and lifetime. There is therefore a need for methods and actuators that significantly increase driving torque, eliminate or reduce effects of stiction, improve production yield, reduce micro-mirror production cost, and increase micro-mirror reliability.